FIG. 1 shows the structure of a known phase-shifted full-bridge DC-DC converter. The phase-shifted full-bridge DC-DC converter includes a switching circuit consisted of four transistor switches Q1, Q2, Q3, Q4. The phase-shifted full-bridge DC-DC converter further includes an isolated transformer T1, a rectifier (D1, D2), and an output filter (Lf, Co). An input DC voltage Vin is converted into an AC voltage by the switching operation of the switching circuit (Q1, Q2, Q3, Q4), and the resultant AC voltage is delivered to the secondary side of the transformer T1. The AC voltage induced across the secondary side of the transformer T1 is converted into a desirable output DC voltage by the rectifier (D1, D2) and the output filter (Lf, Co), in which the output DC voltage is provided to the load 10.
When the transistor switches Q2 and Q4 within the phase-shifted full-bridge DC-DC converter are on, the primary current of the transformer T1 is flowing in a clockwise direction. In this case, the transformer T1 will undergo a positive magnetization process, and the ON-period of the transistor switches Q2 and Q4 is defined as the positive half-cycle of the magnetization process of the transformer T1. When the transistor switches Q1 and Q3 within the phase-shifted full-bridge DC-DC converter are on, the primary current of the transformer T1 is flowing in a counterclockwise direction. In this case, the transformer T1 will undergo a negative magnetization process, and the ON-period of the transistor switches Q1 and Q3 is defined as the negative half-cycle of the magnetization process of the transformer T1. During the magnetization process of the transformer T1, due to the switching characteristics of the switching circuit (Q1, Q2, Q3, Q4), such as the difference between the rising time and falling time of the switching circuit (Q1, Q2, Q3, Q4), the delay difference between the driving circuit of the switching circuit (Q1, Q2, Q3, Q4), and the asymmetry in the circuitry, the period of the positive magnetization process of the transformer T1 does not coincide with that of the negative magnetization process of the transformer T1. This would cause an imbalance between the volt-second product during the positive half-cycle of the magnetization process of the transformer T1 and the volt-second product during the negative half-cycle of the magnetization process of the transformer T1, thereby inducing a DC flux bias in the transformer T1. The DC flux bias of the transformer is prevalently existed in the converter having a transformer needing to be bi-directionally magnetized. The full-bridge DC-DC converter using PWM control technique and the push-pull DC-DC converter are suitable examples of such kind of power converter.
If the DC flux bias of the transformer T1 is not well regulated, the transformer T1 would be saturated. In order to suppress the bias current in the transformer T1, a DC blocking capacitor is placed at the primary side of the transformer T1 to block the bias current. As shown in FIG. 2, a DC blocking capacitor Cb is mounted at the primary side of the transformer T1. When the bias current flows through the DC blocking capacitor Cb, a DC voltage is induced across the DC blocking capacitor Cb. This DC voltage along with the input DC voltage Vin will be provided to the primary side of the transformer so as to carry out the magnetization process of the transformer T1. In this way, the bias current of the transformer T1 is suppressed. Nevertheless, the circuit of FIG. 2 requires an additional DC blocking capacitor Cb compared to the circuit of FIG. 1. In this case, the space occupied by the circuit components on the circuit board will increase and the power density of the power converter will decrease. In addition, the technique for suppressing the DC flux bias proposed by the scheme of FIG. 2 does not use an active manner to control the bias current.
Another solution to suppress the bias current of the transformer is to regulate the peak current of the primary current of the transformer in the positive half-cycle and the negative half-cycle to accomplish the flux bias regulation. According to this solution, the peak current of the primary current of the transformer in the positive half-cycle and the peak current of the primary current of the transformer in the negative half-cycle will be detected and regulated to be identical to the output of a feedback voltage loop (not shown in FIG. 1). Applying this solution to suppress the DC flux bias can ensure that the peak current of the primary current of the transformer in the positive half-cycle coincides with that in the negative half-cycle to be the same. Because the primary current of the transformer is consisted of the magnetizing current and the load current, the regulation over the peak current of the primary current can ensure that the magnetizing current in the positive half-cycle coincides with the magnetizing current in the negative half-cycle. Although this solution does not deteriorate the power density of the power converter, the peak current of the primary current in the current switching cycle has to be detected. Thus, this solution will pose a strict requirement on the real-time accuracy of current detection.
Therefore, it is necessary to propose a novel flux bias regulation method to efficiently suppress the DC bias current in the primary winding of the transformer without the need of a strict requirement on the real-time accuracy of current detection.